General procedure for the synthesis of 4-fluoroindol-2-one from 2-fluoro-6-nitrophenylacetic acid: (2-fluoro-6-nitrophenyl)acetic acid (3.3 g, 16.6 mmol) was dissolved in acetic acid (20 mL) and the hydrogenation reaction was carried out for 24 hours using a 10% palladium-carbon catalyst (0.5 g) under 50 psi hydrogen pressure. Upon completion of the reaction, the catalyst was filtered through Celite to remove the catalyst. The filter cake was washed with methanol and the organic phases were combined and concentrated by evaporation. Subsequently, the concentrate was dissolved in ethanol (100 mL), p-toluenesulfonic acid (50 mg) was added and heated to reflux for 1 hour. At the end of the reaction, the mixture was poured into water and extracted with ethyl acetate, and the organic phase was dried over magnesium sulfate and concentrated by evaporation. Finally, the resulting solid was ground with hexane/ethyl acetate (95/5) mixed solvent to afford 4-fluoro-1,3-dihydro-2H-indol-2-one 1.7 g in 67% yield.
